Last summer on holiday, a mate and I were having a few beers in france. The conversation managed to work it's way round (as it always manages to) to unarmed combat, after discussing his unarmed training from the army, and how it could be expanded, he was eager for me to show him some aikido. So we stand up (swaying slightly after an afternoon of beer), I was just about to open my mouth to explain something when he blurted out "so what would you do against this?" and came at me. I did the first thing that came to mind irimi-tenkan and planted a punch on his cheekbone, down he went.
"Cheers mate, I thought you were going to do a lock or something, I could have figured that out!"

Well a couple of hours, more beers, a bust lip, and a suspected broken nose later I'd managed to show him the basics of aikido, and build on his existing techniques.

Not exaclty a 'DOH!', but I thought I'd highlight the fun of beer fueled debate.